SANTA FE, New Mexico (Reuters) – A municipal ballot measure that would outlaw most late-term abortions in New Mexico’s largest city appeared headed for defeat in a special election by Albuquerque voters, early returns showed on Tuesday.

The measure, were it approved, would bar doctors within city limits from performing abortions after 20 weeks of pregnancy, allowing for few of the exemptions permitted in most late-term abortion bans enacted in other states in recent years.

No exceptions are made for victims of rape or incest. The ban could be waived only to save a mother’s life or if continuing her pregnancy risked “substantial and irreversible physical impairment of a major bodily function” for the mother.

A record number of city voters were reported to have cast early ballots in the special election. And an unusually high overall turnout was expected due to the controversial nature of the measure, which is believed to be the first proposed abortion restriction to be placed on a municipal ballot in the United States.

With results from more than half of the city’s voting centers tabulated, voters appeared on their way toward rejecting the measure, 55 percent to 45 percent, according to figures posted online by the city about 90 minutes after the polls closed.

Public opinion polls suggested sentiment had swung against the Albuquerque initiative since early September, when 54 percent of city voters said they backed the proposal.

Abortion rights advocates had vowed to challenge the measure in court should it pass.

The U.S. Supreme Court legalized abortion nationwide in 1973, but ruled that unless the mother’s health were at risk, states could place restrictions on abortion at the point when a fetus could potentially survive outside the womb, generally seen as starting at 22 to 24 weeks of gestation.

A full-term pregnancy typically is about 40 weeks, and abortions after 20 weeks are rare.

Still, abortion opponents have pushed the boundaries of the landmark Roe v. Wade decision in recent years by seeking to curtail abortions at earlier stages of pregnancy.

The Albuquerque measure was patterned after restrictions enacted by a dozen states based on hotly debated medical research suggesting a fetus feels pain starting at 20 weeks of gestation.

Two of those states, North Dakota and Arkansas, went further by also recently banning abortion as early as six and 12 weeks, respectively. Those more restrictive bans have been put on hold by courts. Courts have likewise blocked 20-week abortion bans in Arizona, Georgia and Idaho.

Albuquerque is home to two of the few facilities in the region that perform late-term abortions – the Southwestern Women’s Options clinic and the University of New Mexico Center for Reproductive Health.

Their existence has led abortion foes to refer to Albuquerque as the “late-term abortion capital of the country” and to target the city for the municipal ban, said Elisa Martinez, executive director of the group Protect ABQ Women and Children, which supports the measure.

Julianna Koob, legislative advocate for Planned Parenthood of New Mexico, agreed that the two clinics had drawn patients from around the region because “access has been so severely impacted in other cities.”

The state attorney general, Gary King, has called the proposed measure “unconstitutional and unenforceable.”

Antoinette Sedillo Lopez, a law professor at the University of New Mexico, said abortion regulations as allowed under Roe v. Wade were regarded as a matter for the states, not local governments, to decide.

Patrick Davis, a spokesman for ProgressNow New Mexico, a non-profit group that supports abortion rights, said approval of the measure in Albuquerque could lead to similar proposals showing up on local ballots in municipalities across the country.

“Using cities to further the culture wars is definitely something we can expect to see in the future,” agreed Lonna Atkeson, director of the Center for the Study of Voting, Elections and Democracy at the University of New Mexico. “This is the first test case.”